As the background art of the present technical field, in Patent Literature 1 for example, a technology of a mechanical-hydraulic type transmission gear applied to a construction machine such as a power shovel is described. In concrete terms, there is described a technology of “a clutch pressure control valve is arranged, in the range where the rotational speed of the output shaft is higher than a predetermined value, the clutch of the mechanical type transmission gear is connected, the clutch of the hydraulic type transmission gear is disconnected, the power is transmitted to the output shaft by the mechanical type transmission gear having excellent transmission efficiency, whereas in the range where the rotational speed of the output shaft is lower than the predetermined value, the clutch of the mechanical type transmission gear is disconnected, the clutch of the hydraulic type transmission gear is connected, and the power is transmitted to the output shaft”.
Also, according to this Patent Literature 1, there is described “at a high speed, the power loss can be reduced because the transmission efficiency is excellent, whereas at a low speed, the output rotational speed between normal rotation and reverse rotation can be controlled smoothly”.